Negative electrode materials for nonaqueous secondary batteries typically include metallic lithium and lithium alloys. The problem associated with these lithium materials is that metallic lithium grows dendritically during charge and discharge to cause an internal short circuit, involving a danger of ignition because of high activity of the dendrite per se. To solve the problem, a fired carbonaceous material capable of reversibly intercalating and deintercalating lithium has been put to practical use. However, the carbonaceous material is disadvantageous in that the capacity per unit volume is low due to a relatively small packing density and that it tends to cause metallic lithium to precipitate thereon in case of overcharge or rapid charge because of its own electrical conductivity.
Other negative electrode materials that have been capable of providing a high-capacity nonaqueous secondary battery having an average discharge voltage as high as 3 to 3.6 V include oxides and composite oxides of Sn, V, Si, B, Zr, etc. (see JP-A-5-174818, JP-A-6-60867, JP-A-6-275267, JP-A-6-325765, JP-A-6-338324, and EP-615296; the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Combined with a positive electrode comprising a certain lithium-containing transition metal compound, these oxides or composite oxides afford a nonaqueous secondary battery having high capacity (an average discharge voltage of 3 to 3.6 V) and high safety, scarcely involving dendrite formation under practical conditions. Further improvement in charge and discharge cycle characteristics has still been demanded, though.
Altering the composition of an electrolytic solution has been attempted as another approach to improvement in charge and discharge cycle characteristics. For example, JP-A-6-333598 teaches addition of a trialkylamine or a triarylamine to an electrolytic solution. This and other proposals, however, are still insufficient for obtaining both high discharge capacity and excellent cycle characteristics where metallic lithium, a lithium alloy or a carbonaceous material is used as a negative electrode material.
Some proposals are found on addition of specific organoboron compounds, which still fail to achieve a satisfactory level of improvement.